If I was going to make one this is how I might do it…
Using investment make a negative mold of a good steel mandrel. Might
be good to lubricate the mandrel first. Might be good to use a smooth
polished mandrel without the size markings. You could mark sizes
after the fact, might be more accurate that way.
After complete curing pour in a suitable material such as the goop
they sell for coating tool handles, might take some experimentation
to find the right substance and the right finished thickness. Pour
out excess material because you don’t want a solid plug. Build up the
thickness in several coats. You could pull the new mandrel after the
first coat or two and then you could test the results as you go.
I might fool around with flexible foam window and door insulation,
the kind that comes in an aerosol can. That might help resiliency.
Squirt some in after you’ve made a hollow mandrel.
Just a thought.